Hall-Heroult aluminum reduction cells have carbon anodes in contact with a molten electrolyte of alumina dissolved in cryolite. Current carrying steel stubs or pins are embedded in the carbon anodes above the bath level. The carbon is prebaked outside of the pot or is baked during cell operation. If prebaked, a pair of steel stubs are inserted in an opening into the top of a carbon block and secured there by pouring molten iron into the space between the stub and the carbon. If baked during operation, a Soderberg cell is used in which vertical steel pins or steel side pins are inserted into green carbon which is progressively baked by the heat of the bath below.
Mild steel has long been the preferred metal of such stubs or pins, because of its strength (especially in forged form), its retention of strength at cell temperatures, its relatively high electrical conductivity, and what has been considered acceptable corrosion resistance. Its relatively low material cost per pound is a favorable factor, but of limited importance.
As noted in U.S. Pat. No. 5,154,813 of Raymond J. Dill, issued Oct. 13, 1992, mild steel stubs corrode particularly where they enter a prebaked carbon anode block, to such an extent as to neck in the stub near its entry and thereby increase electrical resistivity to the point where the stub has to be removed sooner than desired, and replaced instead of reinstalled. Excessive corrosion may also occur where mild steel pins are embedded in the baked carbon of Soderberg anodes. In both cases corrosion may be accentuated through action of sulfates derived from sulfur in the carbon used in the cell.
The Dill patent discloses protecting the mild steel stubs of prebaked carbon anodes by coating them with a corrosion resistant material such as metalized or welded stainless steel. This preserves the advantage of conductivity of the mild steel, which continues to carry the current independently of any conductivity of the coating. However, applying such a coating is an expense to begin with, and is vulnerable to deterioration in the course of first use and subsequent removal, cleaning and reuse to which mild steel stubs and pins are commonly subjected.